When Slack's chat.stopStream fails with user_not_found (Slack Connect DM
recipients), team_not_found (cross-workspace shared channels), or
missing_recipient_user_id (DM closed mid-stream), the text already
delivered via append() is still visible to the user. Swallow those
specific codes and mark the session stopped rather than surfacing a
spurious 'slack-stream: failed to stop stream' error in dispatch. Other
Slack API errors still propagate.
Fixes#70295